The present invention relates to steel, and more particularly, to a ferritic chrome-molybdenum-nickel steel of high chemical resistance against general and intercrystalline corrosion attack, as well as against pitting, crevice and stress corrosion in chloride-containing solutions.
In contrast to the austenitic chrome-nickel-molybdenum steels as the standard materials for the construction of chemical apparatus, it is well-known that highly alloyed chrome-molybdenum steels possess good resistance against general corrosion attack as well as against intercrystalline, crevice and pitting corrosion, and also possess the substantial advantage of excellent resistance against stress corrosion, particularly in chloride-rich and hot solutions.
Equally well known as disadvantages of the conventional ferritic steels, however, are their cold brittleness and their unsatisfactory welding properties.
Although the carbon and nitrogen content of such steels could be reduced to the required low values of less than 0.01% for the first time at the beginning of the 1950's, and the cause of cold brittleness and welding difficulties eliminated by means of the then developing vacuum melting technology, further advances in vacuum metallurgy up to about the end of the 1960's still did not lead to any appreciable volume of production of such decisively improved ferritic steels.
With the development of the new oxygen refining processes for steel melting, worldwide interest in such cold-tough weldable steels has increased steadily since the beginning of the 1970's, enhanced by the ever more urgent demand for chloride-resistant materials, and thereby also the number of new steels that became known. The present state of the art for this new group of materials, the "superferrites", has been presented in "TEW Technische Berichte" (TEW Technical Reports), 2 (1976), pages 3 to 13.
Depending on the desired corrosion properties, the following chrome-molybdenum-(nickel) steels have been mentioned so far in the literature and in patents or patent applications: 18-20/2-3 CrMo; 20/5 CrMo; 26/1 CrMo; 25/4/4 CrMoNi; 28/2 CrMo and 28/2/4 CrMoNi; 29/4 CrMo and 29/4/2 CrMoNi; and 30/2 CrMo.
Depending upon the melting process employed, different low carbon and nitrogen contents are obtained which, on the one hand, affect the cold-strength and the resistance to intercrystalline corrosion (IK) decisively, but, on the other hand, also determine and increase the production costs.
With high chromium contents, total carbon and nitrogen contents of less than or close to 0.01% can be achieved only with elaborate vacuum melting methods, e.g., in an induction furnace or an electron beam cold-hearth furnace. Nickel-free steels melted in this manner need no stabilizing additives of niobium, titanium or the like to ensure IK resistance.
If the more cost-effective melting processes such as VOD (vacuum oxygen refining) and AOD (argon oxygen refining) or their modifications are used, one must tolerate, however, definitely higher carbon and nitrogen contents, depending upon the chromium alloying level. In steels of this type with carbon and nitrogen contents higher than about 0.01%, additions of titanium, niobium or zirconium must be provided against the danger of intercrystalline corrosion; however, the detrimental effect of the increased carbon and nitrogen contents on the cold-toughness properties can be compensated only partially. In a known manner this "stabilization" by means of titanium or niobium brings about a largely stable binding and thereby renders harmless the carbon and nitrogen contents, such that resistance against intercrystalline corrosion, particularly also in the high-temperature zone near welded seams, can be assured without heat treatment.
The possibility of binding the detrimental nitrogen content by the addition of aluminum and thereby improve the cold strength is also known (German Pat. No. 974,555). In addition, an improvement of the resistance against intercrystalline corrosion by the "stable" binding of increased nitrogen contents has been reported in the literature; "Neue Heutte", 18 (1973), pages 693 to 699.
Among the chrome-molybdenum alloy types 25/4, 28/2 and 29/4, variants with additional nickel contents of 2 and 4%, respectively, have become known, whereby the corrosion-chemical behavior was improved considerably and, in addition, the cold-toughness properties were also influenced favorably.
Summarizing the state of the art described in the literature including the pertinent patent literature, highly alloyed ferritic chrome and chrome-molybdenum steels with good mechanical-technological as well as corrosion-chemical properties can contain higher carbon and nitrogen contents with a total above about 0.01% only if these detrimental higher contents are bound stably by additions of titanium, niobium, zirconium or the like and, in the case of nitrogen, also by aluminum, and sufficient cold-strength is ensured, possibly, by a limited further addition of nickel.
In this direction, the steel X 1 CrNiMoNb 28 4 2 (Material No. 1.4575) which is produced on a large scale, is characteristic of the latest state of the art. This steel is a further development of the highly purified vacuum steel X 1 CrMo 28 2 (Material No. 1.4133; see German Offenlegungsschrift 21 53 186), and contains, with about 28% Cr, 2% Mo and 4% Ni, a stabilizing addition of niobium and a total of up to 0.04% carbon and nitrogen.
Complying with a melting specification in this steel 1.4575 of 0.04% (C + N) maximum in large-scale melting, for example, by the VOD process (vacuum oxygen refining), is, however, quite difficult. In addition it has been found that with a chemical composition of this steel with 0.015% maximum C and 0.035% maximum N, or a total of 0.04% maximum (C + N), a limit, not recognized and described in the literature to date, is reached where the niobium content, matched to the carbon and nitrogen contents for steel, for example, with the alloy base CrMiNoNb 28 4 2, cannot be increased further without basic welding problems, and specifically for the reason that the flexibility and elongation of welded joints are made drastically worse in this case.